1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle, and more particularly to an anti-skid control system for controlling a braking force applied to road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
It is known that the vehicle stability or the controllability is influenced detrimentally depending upon the road surface condition, when road wheels are locked in abrupt braking operation. In order to prevent the road wheels from being locked, therefore, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to wheel brake cylinders, and which is also called as an anti-locking control system. The anti-skid control system includes the system for the rear road wheels and that for front and rear road wheels, i.e., four wheel control system. In the case where the road wheels are prevented from being locked according to the former system, the running stability may be ensured and the stopping distance may be minimized. In the case where the front road wheels are also prevented from being locked according to the latter system, the controllability may be maintained.
In view of the fact that, when a hydraulic braking pressure supplied to each wheel brake cylinder is increased, the rotational speed of the road wheel is rapidly reduced immediately before the coefficient of friction relative to the road wheel reaches a maximum, the anti-skid control system controls the wheel cylinder pressure according to the deceleration of the vehicle in order that a slip rate of the road wheel results in around 20%, that is, the maximum coefficient of friction is obtained.
The above anti-skid control system is disclosed in Japanese Patent Publication No. Sho 48-44236, for example, wherein it is determined that a road wheel is in a locking condition, when a decreasing rate of the wheel speed is more than a predetermined value in a rapid braking operation of the vehicle. Then, the anti-skid control is initiated to decrease the hydraulic braking pressure in the wheel brake cylinder. When the decreasing rate of the wheel speed is less than the predetermined value, the hydraulic braking pressure in the wheel brake cylinder is increased until the time when an angular acceleration of the road wheel will reach to the value which corresponds to the angular acceleration memorized during the decreasing mode in the rapid braking operation, thereafter it is increased more gradually comparing with the increasing speed of the hydraulic braking pressure which corresponds to the above angular acceleration.
Thus, in the prior system disclosed in the above publication, when the decreasing rate of the wheel speed is more than the predetermined value, the locking condition of the road wheel is detected, so that the anti-skid control is initiated. Therefore, in the case where the vehicle is running on a rough road, or the road wheel rides on protrusions of the road, or in the case where the wheel speed is momentarily decreased due to backlash of gears in a power train in the anti-skid control system for preventing only driven wheels from being locked, the decreasing rate of the wheel speed may become more than the predetermined value, whereby the locking condition of the road wheel may be erroneously detected to initiate the anti-skid control. Consequently, if the erroneous detection of the locking condition is made immediately after the initiation of braking operation, the anti-skid control will be initiated before the hydraulic braking pressure in the wheel brake cylinder will reach to a sufficient value, and the hydraulic braking pressure in the wheel brake cylinder will be gradually increased, so that the stopping distance will be extended due to lack of braking force.